Wide band gap III-V nitride semiconductors are used for green, blue, and UV light-emitting diodes (LEDs), sensors, and laser diodes (LDs). They are also used for high temperature, high-power, high-frequency electronic devices and nanotechnology.
The most commonly used substrate for III-nitride epilayers is sapphire. Unfortunately, due to the lattice and thermal expansion coefficient mismatches between sapphire and GaN-based materials, a high density of extended defects occur at the GaN/sapphire interface. Cooling from growth temperature to room temperature strains the GaN layer. The overall effect of the residual strain generated on the surface of GaN epilayer grown on the sapphire is a biaxial compressive strain. This strain induced dislocations that originate at the GaN/sapphire interface, and extended vertically with its highest density at the interface.
A high density of dislocations degrades the preparation and operation of the epilayers, heterostructures and quantum-well based optoelectronic devices. Removing the substrate reduces the dislocation density from 108 cm−2 to below 107 cm−2. In addition, sapphire has poor thermal and electrical conductivities. Furthermore, sapphire is difficult to dice since it is hard and does not cleave easily. Hence, it is desirable to eliminate the sapphire substrate and obtain a GaN freestanding membrane that can be used now as a matched substrate for GaN. In addition, the elimination of the sapphire substrate allows the transfer of the heterostructure to a substrate with superior thermal, mechanical, or electrical properties, a requirement for some applications. Removal of the substrate also exposes the backside of the heterostructure, allowing for the implementation of “active packaging” schemes. See M. K. Kelly, O. Ambacher, R. Dimitrov, R. Handschuh, and M. Stutzmann, Phys. Stat. Sol. (a) 159, R3 (1997). Layer transfer also enables the assembly of micro systems.
Laser lift-off (hereinafter “LLO”) technology, of sapphire is commonly used to reduce the dislocation density to below 107 cm−2. The GaN layer is separated from the sapphire substrate and then is used as a freestanding GaN substrate. Though this technology is sophisticated and expensive, it produces GaN substrates with dislocation density of about 106 cm−2 and reduces thermal stresses in devices. Such two-inch substrates are already available and they serve for construction of blue lasers of a power of hundreds of milliwatts.
Decomposition of GaN can result in thermo-mechanical failures. The basic idea of the removal of sapphire from GaN epilayer or heterostructure is that a pulsed laser irradiation causes the decomposition of GaN into gaseous nitrogen (N2) and metallic gallium (Ga) droplets. The thermal and mechanical behaviors during pulsed laser irradiation must be considered for successful LLO and layer transfer (i.e. to obtain damage free GaN lifted layer or freestanding membrane).
Tavernier and Clarke reported that that the N2 pressure can become significant at fluence that are significantly higher than the minimum necessary to induce decomposition. The impact of the high pressure on the LLO mechanism at the GaN/interface could be one or more of the following mechanical constraints: (1) film bulging (2) Fracture of the film or (3) blistering due to tensile failure at the center of a buckle or bulge. Any of these effects constitutes a failure mechanism for most applications of LLO. See P. R. Tavernier & D. R. Clarke, Mechanics of laser-assisted debonding of films, J. Appl. Phys. 89, 31, 1527 (2001), and T. Sands, W. S. Wong, and N. W. Cheung, Laser Liftoff of Gallium Nitride from Sapphire Substrates, available at http://www.ucop.edu/research/micro/98—99/98—133.pdf (last visited Aug. 20, 2007).
However, Tavernier and Clarke reported, “if the laser energy is just sufficient to decompose the film at the interface and cause separation without generating any appreciable N2 vapor pressure, one might expect that film separation can be achieved without any damage.” Tavernier & Clarke, supra. Mechanical failures were observed when Tavernier and Clarke applied fluences of 800 mJ/cm2, 965 mJ/cm2 and 1200 mJ/cm2 to study the mechanical failures. See Sands et al., supra.
Wong et al. stated that at higher fluences above the process threshold, the violent ejection of the sapphire substrate, due to the GaN decomposition, can cause the mechanical fracture of the GaN thin film. W. S. Wong, T. Sands, and N. W. Cheung, Damage-free separation of GaN thin films from sapphire substrates, Appl. Phys. Lett. 72 (5), 2, 599 (1998). They also stated that with fluences greater than or equal to 400 mJ/cm2, successful lift-off was accomplished. However, even using 600 mJ/cm2 fluence, thermally and mechanically damage-free separation of GaN from sapphire was achieved. This indicates that even with fluence that about 30% higher than the threshold, damage free separation can be performed. Accordingly, laser irradiation that is just at the threshold fluence or slightly higher should avoid the mechanical failures. Successful separation can be obtained at fluence of approximately 400 mJ/cm2.
Tavernier and Clarke have performed an extensive study on GaN separation from sapphire. See Tavernier & Clarke, supra. Prior to the LLO the GaN film bonded to a glass substrate using Crystal Bond adhesive as bonding material which based on their experience, is too flexible but is better than many other materials. Three distinct types of behavior were observed when the film was bonded with the Crystal Bond.
First, for an 800 mJ/cm2 pulse, the separation occurred over the area exposed. The film had separated from the substrate and bulging had occurred, but neither lateral cracking nor film cracking was observed. It is believed that the laser pulse induced deformation of the film causes deformation of the Crystal Bond, which then, in turn, constrains the film in its deformed state. Annealing on a hot plate allows the Crystal Bond to soften and the fringes diminish as the film partially relaxes back.
Second, separation of the thin films using pulse of high energy density of 965 mJ/cm2 and GaN film thickness 2 μm and evidence of local inhomogeneities in the laser beam due to multiple mode conditions have been seen. At higher pulse energies ˜1200 mJ/cm2 and a thicker film, extensive cracking of the film accompanied the film separation.
Third, film separation occurred using fluence of 1100 in J/cm2 and the film thickness 2 μm thick. The separated film experiences a lateral crack extends along the interface from the illuminated region, and surrounding the laser decomposed region.
Based on the above study, Tavernier and Clarke have developed detailed processing maps accounting for bulging, buckling, and cracking for GaN films. The maps suggested three strategies to avoid mechanical failures for GaN films during the LLO processing. First, use a beam diameter below the critical size for the onset of cracking a non-bonded film. Second, suppression of cracking by bonding the film as described earlier. Third, start the separation at an edge of the film and move the laser in across the film so that the gas generated by the laser pulse can escape along the separated portions of the film to the edge, thereby lowering the maximum pressure attained.
Nevertheless, Tavernier and Clarke concluded, “ideally, laser-assisted separation of films would be carried out using a laser beam much larger than the size of the film or wafer so that many of the difficulties discussed in their work would be avoided. Physically, the use of a finite beam size restricts the range of laser parameters that can be employed in the separation since the regions of the film not illuminated by the laser constrain, in one way or another, the separation of the film in the illuminated region. Thus, local vaporization of the film causes internal pressures to be generated that can lead to both cracking and bulging deformation of the film.”
Conventional bonding direct and double transfer laser lift-off (LLO). See Timothy D. Sands, Excimer Laser Lift-off for Packaging and Integration of GaN-based Light-emitting Devices, Proceedings of SPIE Vol. 4977, 587 (2003). Sands states that, “Application of LLO to heterogeneous integration requires the use of reasonably thin heterostructures that cannot be easily handled as free-standing films. It is therefore necessary to mechanically support the films throughout the film transfer process. The simplest LLO integration scheme, direct ‘paste-and-cut,’ starts with the permanent bonding of the heterostructure to be transferred to the final substrate. Removal of the substrate by LLO completes the transfer. If the bonding material and substrate are relatively stiff and adhesion at the interfaces is strong, any residual stress in the film will remain after LLO.” Sands, supra.
Sands also reports that, “The direct transfer process described above results in a heterostructure that is inverted relative to its orientation on the growth substrate. In some cases, it may be desirable to transfer a heterostructure in its original orientation.” This led to double transfer process, which Sands stated that “[s]uch a process necessarily involves the temporary transfer of the heterostructure to a handle substrate, a subsequent permanent bonding of the transferred heterostructure to a final receptor substrate, and the release of the heterostructure from the handle substrate.”Sands, supra. However, as reported by Sands “The additional complexities associated with double transfer include the reduced mechanical stiffness associated with the temporary bonding material, which can lead to an increase in problems associated with blistering and crack deflection during LLO.” Sands, supra.